We recently described a novel population of melanocyte-like cells in the pulmonary veins and atria of humans and mice. In the developing and mature heart, melanocyte-like cells are found in a pattern unlike that of any currently known cell lineage and are found in anatomic regions that often give rise to clinical atrial arrhythmia triggers. These cells express a unique transcription signature that is distinct from that of atrial myocytes or dermal melanocytes. Interestingly, isolated murine melanocyte-like cells are electrically excitable and generate atrial myocyte-like action potentials. We have found that genetic deletion of the enzyme dopachrome tautomerase (Dct), which is expressed by both human and murine cardiac melanocytes, unmasks a pathological state with action potential prolongation and afterdepolarizations in these cells. Furthermore, mature Dct knockout mice retain melanocyte-like cells and have structurally normal hearts, yet display increased susceptibility to atrial arrhythmias. While wild-type mice with melanocyte-like cells in their hearts do not have increased atrial arrhythmias at baseline, they do have more atrial arrhythmias when challenged with the muscarinic agonist carbachol compared to c-kit mutant mice that lack melanocyte-like cells in their hearts. In addition, Dct knockout mice have fewer atrial arrhythmias when treated with reactive oxygen species scavengers. Hence, melanocyte-like cells may contribute to atrial arrhythmias in response to increased stresses (i.e. autonomic stimulation or reactive oxygen species) that commonly induce clinical atrial arrhythmias. Despite our initial characterization, the function of melanocyte-like cells during normal physiologic and pathophysiologic states remains obscure. Furthermore, while we have some evidence melanocyte-like cells are excitable and may influence arrhythmogenesis; the underlying electrophysiologic characteristics of these cells require further investigation to understand their potential contribution to arrhythmias. Therefore, we are proposing a series of in vitro and in vivo experiments using genetically engineered mouse models to characterize the cellular electrophysiology of these cells and determine their contribution to atrial arrhythmias. The specific aims proposed include: 1) elucidating the voltage-dependent currents underlying the electrical excitability of isolated murine melanocyte-like cells and the direct contribution of melanocyte-like to atrial arrhythmia triggers, 2) determining the effects of autonomic stimulation upon Dct-positive melanocyte-like cellular excitability and their contribution atrial arrhythmias, 3) assessing the effects of reactive oxygen species upon the excitability of Dct-positive melanocyte-like cells and their influence upon atrial arrhythmias, and 4) investigating the role of melanocyte-like cells in the normal heart. The knowledge we will gain about the basic biology of melanocyte-like cells is likely to open new avenues in our understanding of atrial electrophysiology, with the potential for paradigm shifting insights into the pathogenesis of atrial arrhythmias.